Archived posting to the Leica Users Group, 2006/02/26

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Subject: [Leica] Re:JPEG compression
From: bdcolen at comcast.net (B. D. Colen)
Date: Sun Feb 26 06:33:06 2006
References: <200602251842.k1PIfL4F006362@server1.waverley.reid.org> <e2d1800074ab.4400c25b@optonline.net> <fvb3025rssmbm48ddeh1mhkdihkcp2m996@4ax.com>

If you shoot jpg instea of RAW - a major mistake unless you have no choice 
- immediately upon downloading the image to your computer, save it in a 
lossless TIFF format, and preserve that file as your "negative."
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...... Original Message .......
On Sun, 26 Feb 2006 07:48:09 -0600 "Eric" <ericm@pobox.com> wrote:
"Larry:
"
">If I open a JPEG file in Graphic Converter (for the Mac), make a minor al=
">teration, and save in Best quality, the file size more than doubles. As a=
"> test I down loaded a JPEG file from my wife's P&S camera (a Leica Digilu=
">x Zoom). The file was 197.3 KB. I then saved the file without alteration =
">at the programs's Best quality. The file size was now 412.8 KB.=20
">
">What's going on? What was added to increase the file size?
"
"I'm giong to open a 1000x1500 picture.  That's 1.5 million pixels.  If it's
"24-bit color, that's 3 bytes per pixel, or 4.5M bytes.  Save it in medium
"compression, and you'll probably end up with about a 100k file.
"
"4,500,000 --> 100,000
"
"That's pretty good compression.  The jpeg algorthm is tossing out some bits
"that are so similar to the surrounding bits that we won't really notice the
"loss.
"
"Then, when you load that 100k image, it becames a 4.5M picture because the
"program reconstructs all the tossed out bits.
"
"Let's say you now save at a better quality/lower compression jpeg file.  
You
"end up with a 150k jpeg.  Where did the extra stuff come from?  There is no
"extra size.  It's always a 4.5M picture.  That hasn't changed.  What has
"changed is that instead of getting a 45:1 compression ratio, we're now
"getting a 30:1 ratio.  The lower the ratio, the better the jpeg quality.
"There is little value in re-saving with lower compression, because the 
"made
"up" pixels have already been made up from the first round of
"compressing/uncompressing.
"
"Does this make sense?
"
"
"
"--
"Eric
"http://canid.com/
"
"_______________________________________________
"Leica Users Group.
"See http://leica-users.org/mailman/listinfo/lug for more information


In reply to: Message from lrzeitlin at optonline.net (lrzeitlin@optonline.net) ([Leica] Re:JPEG compression)
Message from ericm at pobox.com (Eric) ([Leica] Re:JPEG compression)